Davidson, et al., describe a correlation microscope in their paper entitled, "An Application of Interference Microscopy to Integrated Circuit Inspection and Metrology" appearing in SPIE Proceedings, Vol. 775, Mar. 1987. Basically, the microscope employs a Linnik microscope in a Michelson interferometer configuration. Each beam then passes through one of two identical microscope objectives and reflects off of different reflecting surfaces, a reference and an object surface. The reflected beams recombine to produce superimposed images at an image plane located at the face of a video camera. In operation, the beam path lengths must be within a coherence length of one another; in addition, both the object and reference surface must be at the focal planes of its respective objective lenses.
If the two beams of light are narrowband, are spatially incoherent and have amplitude A and B, A representing the amplitude of the beam reflected from object and B the amplitude being reflected from the reference mirror, and the beam paths are identical, the output signal is of the form EQU I=A.sup.2 +B.sup.2 +2AB (1)
If the reference mirror is moved sufficiently far so that the two beam paths differ by more than the correlation length, there is no interference between the two beams, and the output signal becomes: EQU I.sub.O =A.sup.2 +B.sup.2 ( 2)
Subtraction of these signals yields an output of the form EQU I.sub.C =2AB (3)
By using electronic processing techniques, it is possible to obtain a product signal or the correlation of the reference and object signals. The image is formed on a TV camera. Processing may be done, for example, by moving the reference plane out of the correlation position and carrying out frame subtraction or using more complicated processes, like Fourier transforming and the spatial filtering the frames taken with different sample plane foci. The simplified treatment given here also gives a rough physical picture of the operation of the microscope when a broadband light source is used.
A system of the type described requires identical objective lenses. The beam path must be matched to a fractional wave length. The microscope elements must be sturdily supported to minimize the effects of vibration. The above adds to the cost and complexity of the correlation microscope.
Another type of interferometer is the Mireau interferometer In this interferometer the reference path and the object path are both in front of a single objective lens. This is accomplished by using two thin pellicles, one of which is used to mount a reference mirror for the reference surface, and the other of which serves as a beam splitter to form the reference and object beams. The advantages of the Mireau interferometer over the use of a Michelson interferometer in the Linnik system are that only one microscope objective is required, so that matched objectives are not needed, and the beam paths are very short, which minimizes the vibration problem. This advantage makes it possible to use a standard microscope support stand, although preferably with vibration isolation. Another advantage of the Mireau interferometer over the Linnik system is that optical alignment is non-critical.
The prior art interferometers are not suitable for use in correlation microscopy because the pellicles are relatively thick, two or more microns, and cause aberration when wide aperture beams are used. Such interferometers are only available for operation with apertures less than 0.5, which severely limits the definition of the system.